It seems rather obvious: When you want the executing assembly, use
GetExecutingAssembly.
Sometimes you don't have one, for example when running as an Office
add-in. You could use Assembly.GetAssembly instead.

I went back and used a UNION statement to give me the correct answer:
select sum(procperf) as 'Procedures Performed'
from (
select sum(proceduresperformed) as procperf
from DriveProjectionAndCollectedTotals DPaCT
inner join rpt_DriveMaster DM on DPaCT.DriveID=DM.DriveID
left outer join rpt_Accounts Acct on DM.AccountID=Acct.AccountID
inner join rpt_AddressDetail AD on Acct.AccountID=AD.AccountID